1. Field of the Invention
The present invention relates to a front light to be used for illuminating a reflective liquid crystal panel or the like, and an electronic device including such a front light.
2. Description of the Related Art
Recently, a larger number of portable devices are provided with reflective LCDs (liquid crystal display devices) as display devices for the following reasons. The reflective LCDs utilize external light for displaying an image and thus do not require a back light which is the most power consuming component in the display device. Thus, by using the reflective LCDs, a portable device driven by a battery can be used over a longer period of time. On the other hand, the reflective LCDs have a drawback in which a bright image cannot be displayed when sufficient external light is not available. In such a situation, the displayed image is not recognized well. In order to overcome the above drawback, a front light has been developed to illuminate a reflective liquid crystal panel when sufficient external light is not available.
FIGS. 13A and 13B show the construction of a prism-type front light as an example of the conventional front lights. This conventional prism-type front light includes a planar light guide plate 1 having a prism surface formed thereon, a light source 2 provided on a side surface of the light guide plate 1, and a reflector 3 for efficiently guiding light emitted from the light source 2 to the light guide plate 1. As the light source 2, a cold cathode tube, an LED or the like can be used.
Operations of the conventional prism-type front light will be described below. When the light source 2 is off (see FIG. 13A), external light 6 from the surroundings is incident on an upper surface 1c of the light guide plate 1 on which a prism is provided, and exits from a lower surface 1d. After reflected from pixel electrodes of a reflective liquid crystal panel 5, the light 6 passes through the light guide plate 1 to reach eyeballs of a user.
FIG. 13B shows operations of the front light when the light source 2 is on. As shown in FIG. 13B, light 8 emitted from the light source 2 is reflected from a lamp reflector 3 to be incident on a side surface 1a of the light guide plate 1. The light incident on the light guide plate 1 is reflected and refracted many times by the upper surface 1c and the lower surface 1d of the light guide plate 1 to be propagated toward the opposite side surface 1b thereof. The above propagation of the light is governed by Snell's law and Fresnel's law. Accordingly, when the light is incident on the interface between air and the upper surface 1c or the lower surface 1d of the light guide plate 1 at an angle smaller than the critical angle, the incident light exits from the lower surface 1d of the light guide plate 1 into air. The transmittance obtainable in the above situation can be calculated in accordance with Fresnel's law. The light exiting from the light guide plate 1 is then incident on the reflective liquid crystal panel 5 to function as illumination light which is effective for providing a display. The light incident on the liquid crystal panel 5 is modulated by the liquid crystal therein, and reflected from the pixel electrodes to be again incident on the lower surface 1d of the light guide plate 1. The light then exits from the upper surface 1c to reach eyeballs of a user.
The above-mentioned prism-type front light is described in many articles, for example, in the article entitled “Front light techniques which expand a range of applications of reflective color liquid crystals” in Liquid Crystal Display Seminar '98, Material Technology Text, E-6(4); the article entitled “Sony has presented its reflective low-temperature poly-Si TFT-LCD” in Monthly FPD Intelligence, (February 1998), p. 22; the article entitled “Reflective color LCD panels appear at EDEX'98—toward full-scale popularization” in Nikkei Electronics, No. 717 (Jun. 1, 1998), pp. 41–46; and the article entitled “Front lights for reflective LCDs based on light guides with micro-grooves” in 1999 SID Symposium Digest of Technical Papers, p. 912.
In the prism-type front light, the total reflection condition at the lower surface of the light guide plate is not satisfied by provision of the concave-and-convex configuration on the lower surface. Alternatively, it is possible to allow the light guide plate to be in contact with a material having a refractive index different from that of the light guide plate so that the total reflection condition is not satisfied there. The latter configuration is not classified into a front light, but used in a back light of ink dot type. On a lower surface of a light guide plate for an ink dot type back light, white ink is printed in dots on the lower surface of the light guide plate. Light incident on these dots are scattered there. The thus-scattered light is allowed to exit from the light guide plate since an incident angle thereof with respect to the upper surface of the light guide plate is smaller than the critical angle. The amount of the light exiting from the upper surface of the light guide plate is set to be uniform over the entire upper surface of the light guide plate by optimizing a size, a pitch, a density of the dots, or the other parameters.
However, the conventional prism-type front light has a drawback of low light utilization efficiency. Since the front light is typically combined with the reflective LCD, the front light requiring a large power consumption for its operation will have an adverse effect on the most advantageous feature of the reflective LCD, i.e., a low power consumption.
The reasons for the low light utilization efficiency can be described as follows. First, a portion of light incident on the prism surface is refracted as shown in FIG. 13B, resulting in light 11 exiting from the upper surface 1c of the light guide plate 1. The light 11 becomes a loss since it does not illuminate the liquid crystal panel, thereby leading to reduced light utilization efficiency. In order to compensate for the resultant reduction in luminance, power consumption of the light source has to be increased. In addition, the light 11 exiting from the upper surface 1c and traveling toward the user is not used for providing a display. Accordingly, recognition of the light 11 by the user will lead to a decreased contrast.
Secondly, the light entered into the light guide plate 1 cannot easily exit therefrom through the lower surface 1d, and therefore, is likely to be lost in the light guide plate 1. This in turn leads to reduced light utilization efficiency and lower luminance. More specifically, the light incident on the side surface 1a of the light guide plate 1 at a small incident angle experiences the smaller numbers of reflection and refraction at the upper and lower surfaces 1c and 1d, so that the light is likely to satisfy the total reflection condition. When the total reflection condition is satisfied, the light continues to be propagated in the light guide plate 1, while repeating reflections, to be finally attenuated therein.
As the third reason, the light emitted from the light source 2 is likely to exit from the light guide plate 1 toward the LCD at a large angle (i.e., an angle between the light and the normal to the lower surface 1d of the light guide plate 1 is likely to be large). This is because only the light incident on the lower surface 1d of the light guide plate 1 at an angle smaller than the critical angle for the total reflection can exit through the lower surface 1d. 
While the light is propagated in the light guide plate 1, an incident angle to the lower surface 1d becomes gradually smaller. When the incident angle to the lower surface 1d becomes slightly smaller than the critical angle for the total reflection, the total reflection condition is not satisfied and the light exits from the lower surface 1d of the light guide plate 1 into air. Accordingly, the exiting angle in this situation is close to 90°. Such light is not allowed to be incident on the reflective liquid crystal panel 5 at the right angle, thereby resulting in reduced light utilization efficiency.
A projection-type front light as shown in FIGS. 14A and 14B is intended to overcome the above-explained disadvantages of the prism-type front light. This projection-type front light includes a light guide plate 21, a light source 22, and a reflector 23. A lower surface 21d of the light guide plate is formed to have projections with a rectangular cross-section.
When the front light is not on, as shown in FIG. 14A by an arrow, the external light incident on an upper surface 21c of the light guide plate 21 passes through the light guide plate 21 to illuminate a reflective liquid crystal panel 25. The light reflected from the reflective liquid crystal panel 25 reaches eyeballs of a user.
When the front light is on, as shown in FIG. 14B by an arrow, the light emitted from the light source 22 is reflected from the reflector 23 to be incident on a side surface 21a of the light guide plate 21. The incident light is propagated in the light guide plate 21 toward the opposite side surface 21b thereof while being totally reflected between the upper surface 21c and the lower surface 21d. Of the light being propagated within the light guide plate 21, portions incident on the upper surface 21c are likely to satisfy the total reflection condition. Accordingly, little light can exit through the upper surface 21c. In addition, of the light incident on the lower surface 21d, portions incident on the bottom surfaces 24a of the convex portions and the bottom portions 24b of the concave portions always satisfy the total reflection condition. Accordingly, no light can exit from the light guide plate 21 through the bottom surfaces 24a of the convex portions and the bottom portions 24b of the concave portions.
On the other hand, the light incident on the side surfaces 24c of the convex portions can pass therethrough since the incident angle thereof becomes smaller than the critical angle. As can be understood from the above, little light can exit through the upper surface 21c of the light guide plate 21 in the projection-type front light, thus a loss of light becomes smaller as compared to the prism-type front light.
Furthermore, as shown in FIG. 15, a front light having projections 34 provided on a lower surface of a light guide plate 31 to have a trapezoidal cross-section. The front light in FIG. 15 can operate in the manner similar to the front light in FIGS. 14A and 14B, and furthermore, the light is allowed to pass through side surfaces 24c of the convex portions by providing projections on the light guide plate 31 with a reverse-tapered cross-section. In FIG. 15, the same components as in FIG. 14 are designated by the same reference symbols.
The above-mentioned projection-type front light is described, for example, in the article entitled “A front-lighting system utilizing a thin light guide” in ASIA DISPLAY '98, p. 897. The advantage of the projection-type front light is to overcome the above-described first disadvantage of the prism-type front light. While the light emitted from the light source exits through the upper surface (i.e. through the side closer to the user) in the prism-type front light, only the light incident on the side surfaces 24c of the projections can exit from the light guide plate in the projection-type front light, thereby resulting in decreased light loss and suppressed reduction in contrast.
It should be noted that as shown in FIGS. 14A and 14B, the light incident on the side surfaces 24c of the projections is used for illuminating the reflective liquid crystal panel 25. However, the disadvantage relating to a large incident angle to the reflective liquid crystal panel 25, which is derived from the large exiting angle from the side surface 24c of the convex portion, has not been still overcome. The large incident angle means that the light is incident on the pixel electrodes from the oblique direction, resulting in lowered light utilization efficiency. Furthermore, since only the light incident on the side surfaces 21c of the convex portions can exit from the light guide plate 21, it is difficult for light to exit from the light guide plate 21. Accordingly, the light is still likely to be lost at the high probability during the propagation, and the disadvantage relating to this point has not been yet overcome.